Hereinafter, a “Q” prefix in a word of phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1 s and 0 s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a qubit, the Josephson junction—which functions as a dispersive nonlinear inductor—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the inductance and the capacitance in the qubit circuit. Any reference to the term “qubit” is a reference to a superconducting qubit circuitry that employs a Josephson junction, unless expressly distinguished where used.
The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a q-processor to perform computations.
A superconducting qubit has two quantum states—|0> and |1>. These two states may be two energy states of atoms, for example, the ground (|g>) and first excited state (|e>) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states are allowed and valid.
For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not alter the energy states of the qubit, such as by injecting or dissipating energy, in any significant manner or influence the relative phase between the |0> and |1> states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits.
A reset of a qubit is the process by which the qubit's energy state returns from an excited state to a ground state. A time constant (T1) characterizes the exponential decay versus time of the excited energy state of the qubit to the ground state.
In general, a superconducting quantum interference device (SQUID) is used as a very sensitive magnetometer that can measure extremely low magnetic fields. SQUIDs are sensitive enough to measure fields as low as 5 atto Tesla (5×10−18 T). For comparison, a typical refrigerator magnet produces 0.01 tesla (10−2 T).
There are two main types of SQUID: direct current (DC) SQUID and radio frequency (RF) SQUID.
A dc-SQUID is based on the DC Josephson effect and has two Josephson junctions in parallel in a superconducting loop. In the absence of any external magnetic field, the input current splits into the two branches—one to each Josephson junction in the loop—equally. If a small external magnetic field is applied to the superconducting loop, a screening current, begins circulating in the loop that generates a magnetic field canceling the applied external flux. The induced current is in the same direction as in one of the branches of the superconducting loop, and is opposite to in the other branch; the total current becomes in one branch and in the other branch. As soon as the current in either branch exceeds the critical current Ic of the Josephson junction in that branch, a voltage appears across that junction. If the external flux is further increased until it exceeds, half the magnetic flux quantum, because the flux enclosed by the superconducting loop must be an integer number of flux quanta, instead of screening the flux the SQUID now energetically prefers to increase it to towards a flux quantum. The screening current now flows in the opposite direction. Thus, the screening current changes direction every time the flux increases by half integer multiples of flux quantum. Thus, the critical current oscillates in the superconducting loop of the dc-SQUID as a function of the applied flux.
The illustrative embodiments recognize that presently, a significant amount of time is wasted in waiting for the qubit to reset. Generally, for long-lived qubits the decay of the excited state to the ground state is slow. Consequently, the reset operation that is based on waiting for the qubit to decay to the ground state is also slow. The longer the T1 of the qubit the longer is the idle time.
The illustrative embodiments recognize that this wastage of time has a direct adverse effect on the speed of quantum computations that are possible using superconducting qubits. The illustrative embodiments further recognize that because the commonly used method for reset is passive, i.e., not performed using circuit other than the qubit itself, other quantum circuits, such as the readout circuit that is presently employed for reading qubits, play no role in resetting the qubit.
The illustrative embodiments recognize that an active method of resetting the qubit is therefore desirable. In the active method, a quantum circuit external to the qubit operates in a manner to force the qubit to the ground energy state, accelerates the decay of the qubit to the ground energy state, or some combination thereof. The illustrative embodiments further recognize that a quantum circuit that is capable of multiple operations, such as both readout and reset operations on a qubit, is also highly desirable.